As tensions between the North and South increased in the mid-19th century, Congress became increasingly divided on the issue of slavery. Southern representatives threatened to secede from the Union if the federal government attempted to interfere with slavery in their states. In an effort to prevent secession, Congress passed the Fugitive Slave Act of 1850, which required Northern states to assist in the capture and return of escaped slaves.
Despite these compromises, tensions continued to mount between the North and South. In 1860, Abraham Lincoln was elected president on a platform opposed to the expansion of slavery into new territories. His election led to the secession of 11 Southern states and the outbreak of the American Civil War.
The Civil War ended in 1865 with the victory of the Union. As a result of the war, slavery was abolished throughout the United States by the Thirteenth Amendment to the Constitution. Congress took a leading role in this process by passing the Emancipation Proclamation and the Thirteenth Amendment, and by supporting the Union war effort.